kontext

Kontext

Overview

This repository contains an EXPERIMENTAL PROTOTYPE, with the goal of uploading a local source context (e.g. a local directory) for use in the Knative Build CRD (or compatible definitions).

The high-level objectives of this prototype are:

1. Aim for a level of incrementality akin to the "GCS Fetcher" currently available in the Knative Build CRD,
2. Work through vendor-agnostic / standardized APIs,
3. Don't require implementation-specific knowledge of how the Build CRD works.

How it works (tl;dr)

This tool works by essentially creating a self-extracting source-context, which when run expands the files collected from the local directory into /workspace.

How it works (detail)

Similar to ko, this tool assembles a container image directly (without Docker), which effectively looks like:

# This effectively does a: cp -r /var/run/kontext /workspace
FROM <image pre-built from ./cmd/extractor>

# Add the directory containing the desired context under this path.
ADD ./dir /var/run/kontext

# Add a manifest of the files included (more on this later)
ADD <manifest> /var/lib/kontext/manifest.json

So without Docker, we effectively assemble the source directory into a container image, which can be done by invoking:

go run ./cmd/kontext/main.go --directory=/path/to/directory --tag=gcr.io/project/image:tag

This can then be supplied to the Build CRD's custom source step like:

spec:
  source:
    custom:
      image: gcr.io/project/image:tag

Or tested locally via:

docker pull gcr.io/project/image:tag
docker run -ti --rm -v /path/on/host:/workspace gcr.io/project/image:tag

How it is incremental

On subsequent invocations, kontext first cracks open the target image (if it exists) and extracts the manifest file, which reflects the state of the /var/run/kontext directory once mounted by an overlayfs. Leveraging this context, it can compute a "delta" layer, which only contains changed files. For example:

FROM <previous iteration of the target image>

# Based on the manifest of the previous image, a single layer
# is added that adds any changed files, and "whites out" any
# files/directories that have been removed.
ADD <ONLY CHANGED FILES> /var/run/kontext

# Add a new manifest of the files included.
ADD <manifest> /var/lib/kontext/manifest.json

So for example, if the directory being uploaded as context is:

a/
  b
  c       <-- new
  d

foo/
  bar     <-- new

baz/
  blah/
    bleh  <-- changed

gone/
  ...     <-- all removed.

Then the "delta" layer would only contain:

a/
  c

foo/
  bar

baz/
  blah/
    bleh

.wh.gone  <-- "whiteout" file

TODOs

• Handle layer limits
• Handle symlinks
• Heuristics around whether to reuse the previous image at all